Oil and gas refineries frequently use systems of pipes and tubes (hereinafter “tubes”) in processing fluids. In many applications, tubing is used not only as a conduit for transporting fluids to be processed, but also as an integral component of the chemical plant processing equipment, for example, cracking furnace tubes.
Cracking furnaces of the prior art generally comprise a refractory lined firebox containing a multiplicity of highly alloyed (metallic) reactor furnace tubes through the interior of which flows the hydrocarbon feedstock to be cracked, together with a suitable amount of dilution steam. The sensible heat and the heat of cracking are supplied primarily by radiant heat from burners located on the floor and/or walls of the firebox. This heat transfers through the alloy reaction lines (reactor furnace tubes) into hydrocarbon feedstock that flows there within to provide the necessary energy for cracking the hydrocarbons.
Moreover, cracking furnaces, as constructed today, provide for millisecond residence time at a maximum bulk fluid temperature of about 1625° F., and are, with respect to their radiant heated reactor furnace tubes, constructed of metallic materials. The fireboxes themselves, which may be lined with refractory materials, are capable of delivering a greater heat load than the metallic materials of the reactor furnace tubes can withstand. This maximum service temperature of the metallic materials, of which the reactor furnace tubes are constructed, limits the performance of the aforementioned reactor furnace tubes with regard to their capacity (which should be as high as possible), and their residence time (which should be as short as possible) and, hence, selectivity (to achieve the highest possible yield of valuable olefinic product species like ethylene and propylene, for example).
Given the relatively high temperatures to which the reactor furnace tubes are exposed in a thermal cracking process, metallic materials have been the preferred materials for construction of such tubes. As reactor designers have strived for the higher capacity and higher selectivity in the process, which would result from the use of materials with higher maximum service temperature limits, they have steadily improved the properties of the metallic alloys from which the reactor furnace tubes are manufactured.
Conventional reactor furnace tubes are constructed of nickel-containing alloys, the majority of which are prepared from compositions comprising chromium, nickel and iron in the range of 18 to 38 weight percent chromium, 18 to 48 weight percent nickel, the balance iron, i.e., steels, and alloying additives. These alloys are used in industrial processes that operate at elevated temperatures generally above 1100° F. and up to 2000° F., or more. In general, the development of the nickel-containing alloys for reactor furnace tubes, in order to increase the maximum service temperature of the aforesaid reactor furnace tubes, has been accomplished by the careful control of composition and microstructure to produce, for example, improved quality austenitic nickel-chromium steels. See, for example, Kleeman, U.S. Pat. No. 6,409,847 (the contents of which are incorporated herein by reference). The best nickel-containing austenitic steels and alloys, however, still have maximum service temperatures of only around 2100° F.
At high cracking temperatures, however, the nickel in conventional reactor furnace tubes acts as a catalyst for coke formation inside the tube—a particular form of coke that is termed “catalytic coke.” Coke also forms on the walls of the metal tubes as the result of the pyrolysis itself, i.e., the action of time and temperature (particularly the very hot wall temperature) on the coke precursor material produced in the reactant mass. This type of coke, having both a different formation mechanism and a different structure from catalytic coke, is known as “pyrolytic coke.” The coke formed by pyrolysis overlays on top of the catalytic coke in the reactor furnace tube. The deposition of pyrolytic coke, being a function of time, temperature and coke precursor material, increases in amount along the tube length, peaking at the output end of the reaction tube where time, temperature and precursors are at increased levels. For recent examples of a general discussion of coke formation in the cracking field, see, for example, the following: “Kinetic Modeling of Coke Formation during Steam Cracking”, S. Wauters and G. B. Marin, Industrial & Engineering Chemistry Research, 41 (10), 2379-91; Comments on “Kinetic Modeling of Coke Formation during Steam Cracking,” Lyle F. Albright, Industrial & Engineering Chemistry Research, 41 (24), 6210-12; and Reply to Comments on “Kinetic Modeling of Coke Formation during Steam Cracking,” Marie-Francoise S. G. Reyniers, Sandra Wauters, and Guy B. Marin, Industrial & Engineering Chemistry Research, 41 (24), 6213-14.
Coke formation is deleterious to the process for a number of reasons. The deposition of coke on the insides of the reactor furnace tubes constricts the flow path for the hydrocarbons, causing an increased system pressure drop and reduced throughput. The higher average hydrocarbon partial pressure reduces the selectivity of the process; and in extreme cases, the coke can cause maldistribution of flow (between parallel reactor furnace tubes) and, ultimately, a decrease in the furnace capacity. Additionally, the coke lay-down on the inside of the furnace tubes increases the resistance to heat transfer between the outside of the reactor tube wall and the bulk fluid flowing within the reactor tube. Consequently, the outside flue gas temperature, the firing rate and the outside tube wall temperature have to be increased in order to maintain the same temperature and/or conversion of the hydrocarbon fluid flowing within the tube. Eventually the outside temperature of the wall of the reactor tube can reach the maximum service limit for the material from which the tube is manufactured, under which circumstances the furnace needs to be shut down and the coke removed by passing a mixture of steam and air through the tubes in order to convert the coke (basically carbon) to a mixture of carbon oxides. This process is known as “decoking.” Decoking consumes valuable resources and, in the case of conventional nickel-containing metallic alloy reactor furnace tubes, reduces the life of the tubes. Tube life is reduced by a variety of mechanisms including, but not limited to, abrasion, thermal fatigue, and damage to the internal oxide protective layer.
Attempts to reduce coking by varying the materials used for reactor furnace tubes are found in the prior art. For example, the prior art describes the use of silicon ceramics for reactor furnace tube construction. For example, Winkler et al., U.S. Pat. No. 2,018,619, describes an apparatus for the pyrogenic conversion of hydrocarbons that uses reaction tubes made from silicon powder; Endter et al., U.S. Pat. No. 2,987,382, describes a furnace for carrying out gas reactions in ceramic tubes; Coppola et al., U.S. Pat. No. 4,346,049, discloses silicon carbide powder compacts for forming furnace tubes; and Williams et al., U.S. Pat. No. 5,254,318 describes lined tubes for high pressure reformer reactors. European Patent Application EP 1 018 563 A1 discloses a heating furnace tube comprising a rare earth oxide particle dispersion strengthened (ODS) iron alloy containing 17-26 wt. % of Cr and 2-6 wt. % of Al and a method for using and manufacturing such a heating tube in locations where the coking and carburization problems occur during the process.
More recent innovations to furnace tube compositions include more temperature resistant, non-nickel containing materials, such as, ceramics and/or oxide dispersion-strengthened (“ODS”) alloys for use in cracking hydrocarbon feedstock at temperatures of 1300° F. or higher, see for example U.S. patent application No. 2004/0147794, the entire contents of which are incorporated herein by reference.
The various tubes used in chemical plant processing equipment have different material requirements. For example, reactor furnace tubes located within the radiant section of the furnace are required to tolerate temperatures above the cracking temperature of the feedstock. It is very important only to have the feed at a temperature above cracking for a specific amount of time in order to prevent overcracking or non-selective cracking. Furnace tubes, however, are connected to other tubes, such as, transfer and cross-over tubes for transporting the hydrocarbon gases. Because cross-over tubes and transfer tubes are not exposed to as much heat as the furnace tubes are exposed to, their composition can be quite different than that of the furnace tubes. The use of mixtures of various materials, such as, ceramics, metallic and the like, in order to provide furnace tubes that can withstand higher temperatures, increase capacity and higher selectivity, has invariably lead to the need for joining together ceramics and metallic alloys in various processing equipment components.
Moreover, the joining of metals/alloys and ceramic materials is problematic. Joining can be considered as the creation of a controlled interface between the two components or materials to be joined. It is important to control the interface for two main reasons: first, to ensure that appropriate or advantageous chemical reactions occur (to ensure wetting and bonding in a brazing operation, for example, or to provide sufficient diffusion in a diffusion bond); and, second, to negate, if possible, the differences in coefficient of thermal expansion (CTE).
For example, the joining of silicon carbide (SiC) and iron chromium nickel alloys fails because the materials react in a deleterious manner (at temperatures above about 2000° F.) to form relatively low melting point Ni-silicides. Such reactions could take place during bonding procedures such as brazing or diffusion bonding, or during service. Additionally, these materials have a very different co-efficient of thermal expansion (CTE). If the two materials were brought directly together and joined (by brazing or diffusion bonding, for example), there is a high likelihood of failure (if the bond area is any more than a few square millimeters).
There are various ways known to overcome the effects of CTE mismatch. These include the use of a single interlayer, double interlayers and flexible interlayers. These interlayers being bonded between the two primary components. Fully graded or functionally graded materials or interlayers have also been proposed. Fernie et al. Welding and Metal Fabrication, 5 (1991) 179-194.
Although combining two different materials, for example, advanced (i.e., technical) ceramics and metal alloys to produce functionally graded materials (FGM) is known in the art, see for example, Pietrzak et al. Journal of the European Ceramic Society, 27 (2007), pgs. 1281-1286 and Ruys et al., Journal of the European Ceramic Society, 21 (2001), pgs. 2025-2029, the methods disclosed in the prior art have met with some utility, but suffer from drawbacks. Similarly, the production of functionally graded materials based on two ceramics has been demonstrated, C. S. Lee, X. F. Zhang and G. Thomas: ‘Novel joining of dissimilar ceramics in the Si3N4—Al2O3 system using polytypoid functional gradients’, Acta Mater., 2001, 49, 3775-3780.
Thus, there remains a need within the industry for joining different materials having dissimilar coefficients of thermal expansion, such as advanced ceramics with metallic materials, to provide improved tubes, for use in, e.g., chemical plant processing equipment.